During the manufacturing as well as the operation of micro electromechanical systems (MEMS) devices, it is well known that failure due to adhesion between surfaces, e.g. between a moving surface and a substantially stationary surface, of the device may occur. This phenomenon is referred to as stiction. Stiction occurs with a larger probability in microstructures, typically having dimensions in the order of magnitude of 1-3 μm because the surface-to-volume ratio increases and surface forces, which are responsible for stiction, are correspondingly higher. Stiction may occur during or after the manufacturing process (i.e. during operation), after releasing of the microstructure where the surface tension of the rinse liquid is sufficiently strong to pull the suspending microstructures in contact with the substrate or another compliant or stiff counter surface, leading to permanent adhesion. This kind of stiction is referred to as ‘after-release stiction’. Alternatively or additionally, stiction may occur after a successful release, e.g. when a microstructure is exposed to an environment of increased humidity or changing temperature. If the microstructure is first exposed to a humid environment, water vapour can condense and form a water film/droplets on the device surfaces. When the distance between the two surfaces decreases during device operation and the water film/droplets of one surface touch the counter surface, the two surfaces will stick together. This phenomenon may occur during the normal device operation and is therefore referred to as ‘in-use stiction’. In-use stiction is in particular a problem in microstructures in which opposite surfaces, e.g. a diaphragm and a back-plate, form capacitors in combination with each other. This is, e.g., the case in condenser microphones and condenser pressure sensors.
The present invention is concerned with preventing stiction in microstructures, in particular in MEMS condenser microphones.
It is further known that the application of a hydrophobic layer to the surfaces in question can solve, or at least relieve, the problem. This has, e.g., been described in U.S. Pat. No. 5,822,170, in “Anti-Stiction Hydrophobic Surfaces for Microsystems” by P. Voumard, et al., CSEM scientific and technical report 1998, Neuchâtel, Switzerland, 26, in “The property of plasma polymerized fluorocarbon film in relation to CH4/C4F8 ratio and substrate temperature” by Y. Matsumoto, et al., Proc. of Transducers '99, Jun. 7-10, 1999, Sendai, Japan, 34-37, in “Self-Assembled Monolayer Films as Durable Anti-Stiction Coatings for Polysilicon Microstructures” by M. R. Houston, et al. Solid-State Sensor and Actuator Workshop Hilton Head, South Carolina, Jun. 2-6, 1996, 42-47, in “Elimination of Post-Release Adhesion in Microstructures Using Conformal Fluorocarbon Coatings” by P. F. Man, et al., Journal of Microelectromechanical Systems, Vol. 6, No. 1, March 1997, in “Anti-Stiction Methods for Micromechanical Devices: A Statistical Comparison of Performance” by S. Tatic-Lucid, et al., Proc. of Transducers '99, Jun. 7-10, 1999, Sendai, Japan, 522-525, in “A New Class of Surface Modification for Stiction Reduction”, by C.-H. Oh, et al., Proc. of Transducers '99, Jun. 7-10, 1999, Sendai, Japan, 30-33, in “Self-Assembled Monolayers as Anti-Stiction Coatings for Surface Microstructures”, by R. Maboudin, Proc. of Transducers '99, Jun. 7-10, 1999, Sendai, Japan, 22-25, and in “Anti-Stiction Silanization Coating to Silicon Micro-Structures by a Vapor Phase Deposition Process”, by J. Sakata, et al., Proc. of Transducers '99, Jun. 7-10, 1999, Sendai, Japan, 26-29.
The references above describe depositions of a hydrophobic layer, e.g. a self-assembled monolayer (SAM) onto surfaces of the microstructure, the microstructure preferably being made from a silicon material, such as a Si-wafer or poly-silicon layers. The deposition is primarily performed by successively positioning the microstructure in various liquids. However, in “Anti-Stiction Silanization Coating to Silicon Micro-Structures by a Vapor Phase Deposition Process”, by J. Sakata, et al., Proc. of Transducers '99, Jun. 7-10, 1999, Sendai, Japan, 26-29, the deposition is performed by a vapour phase deposition process (dry process), in which the microstructure is positioned in a container containing a gas or a vapour. The advantage of this process is that it is possible to obtain a homogeneous coating, even inside a complicated microstructure, and even inside a space with narrow gaps. However, it has turned out that using a vapour phase deposition process results in a hydrophobic layer having a surface which is less structured than the surface of a hydrophobic layer which has been deposited using a liquid phase deposition process. This is due to the fact that the molecules forming the monolayer form cross bindings in addition to forming bonds to the surface. With a certain probability, this reaction already happens in the gas-phase. Therefore, molecule clusters are deposited that cannot chemically bind to the surface anymore or that can only partly chemically bind to the surface. This results in a less structured layer and therefore rough surface, which makes it possible for water droplets to attach to the surface, even though the material surface otherwise would be highly hydrophobic. Thus, the hydrophobic property of the surfaces is partly or possibly totally reduced. Furthermore, the process described in this reference requires special equipment. In addition, the sacrificial layer has to be removed and the structure has to be released before the hydrophobic layer can be applied. The release process is a critical process with a certain yield, which will reduce the total yield of the manufacturing process and increase the manufacturing costs. The gas phase deposition also needs pumping steps, which bear the risk for stiction due to fast pressure transients. Therefore, the coating process performed from a liquid material is preferred.
It is, thus, desirable to be able to provide a method for providing a hydrophobic layer to the inner parts of a microstructure in such a way that the hydrophobic property of the layer is maintained.